This is an ancillary study of the Phase II multicenter, randomized, placebo-controlled trial of hydroxyurea therapy for sickle cell disease. Both UNC and Duke are taking part in this trial. During Phase I of the study, 26 of the 32 patients treated gained weight. This study will make use of the opportunity provided by a Phase II study to investigate the etiology, tissue composition, and any change in muscle function that accompanies weight gain associated with HU therapy.